A recurrent problem in the manufacture, testing, installation, and operation of rotating machinery is the need to balance the rotor. The balancing procedure involved is usually referred to as field balancing, trim balancing, or in-place balancing when performed in the test cell or in the final installation. Factory balancing in traditional low-speed machines usually involves general sensors (integral parts of the balancing machine) used to identify balance corrections in one or at most two locations along the rotor's shaft axis. However, in many devices in which rotors operate at high speeds, the ability and accuracy in determining and correcting vibrations due to an unbalanced rotor becomes more and more critical to satisfactory operation of the device. An important example of this is in commercial and military jet engine applications which utilize a single stage rotor or multiple-stage rotors. Periodically for safety and efficiency, the jet engine undergoes testing, and if necessary overhauling. The system testing involves determination of the actual levels of critical performance and operating parameters versus the allowable levels. Frequently, engine vibration levels which exceeds the allowable limits are detected. The rotor therefore must be trim balanced, that is, the synchronous vibration components of a trim balanceable rotor are reduced to a permissable level. If the rotor cannot be balanced by trim balancing techniques (i.e., due to lack of access to the area in need of trim balancing, rotor shift, misalignment etc.), the whole engine is returned to the final assembly area for corrective rework.
Such situations are true not only in jet engine applications but also in many others which involve complex machines and require a high degree of accuracy in the balancing of the rotor.
Regarding particularly however the example of jet engines, present procedures required for correcting excessive rotor vibrations are time consuming and lacking in precision. Trim balancing, a simple operation compared to finding and correcting other vibration problems, still takes approximately five hours. In this regard, if during the testing of the engine and rotor, it is found to have excessive vibrations, no matter where they originate, the operator will attempt to trim balance the engine and rotor by adding weights to the accessible portion of the rotor. The engines that cannot be trim balanced in this fashion are rejected and returned to the rework area. However, much time and effort is wasted in attempting to trim balance engines which, due to the nature of the unbalance, are not capable of being trim balanced. Moreover, since the nature of the unbalanced condition is not sufficiently ascertained, only a minimum amount of data is available to direct engine rework. Moreover, what data is available is determined by a testing operator and, therefore, subjective and highly dependent upon the experience of the operator. This results in additional and unnecessary time spent on reworking the engine to enable it to meet acceptable operating standards.
The already high cost of rotor testing and overhaul is ever increasing. Also, the present and increasing requirement for rapid turnaround of overhauled engines, particularly jet engines, has created a need for reducing the time and cost to test and overhaul engines, thereby increasing the availability of the engines for operation.